It is known to provide bearings such as ball bearings or roller bearings with temperature sensors. For monitoring the bearing, heat generated at the interface of balls/rollers and the raceways is detected by a temperature sensor located as close as possible to this interface. This makes a cage of a bearing a desirable position for temperature measurement. However, a bearing cage is a rotating part and cannot be easily accessed and the available space is narrow. In particular for smaller bearings, it is difficult to integrate batteries or a generator into the bearing cage.
The document EP 1 849 013 B1 discloses a bearing having a cage equipped with a power receiving coil. A transmitter for sending a signal indicative of the sensed condition of the bearing to another antenna arranged on the outer race is integrated in the cage. However, this solution is feasible only for larger bearings with a cage capable of supporting the transmitter electronics.
Passive wireless temperature sensors for bearing cages have been proposed e.g. in the article “A Passive Wireless Temperature Sensor for Harsh Environment Applications”, Sensors 2008, 7982-7995 by Ya Wang, Yi Jia, Qiushui Chen and Yanyun Wang. A passive oscillator circuit including a temperature dependent capacitance and an inductor can be arranged on a rotating element like a bearing such that the inductor coil interacts with a transmitter coil of a driven oscillator circuit. The temperature dependence of the capacitance results in a temperature dependence of the resonance frequency of the resonant circuit on the rotating element which can be measured. Similar systems have been investigated by S. Scott et al of the Purdue University.
The resonant circuit absorbs energy from the primary circuit with a coil such that the resonant frequency can be derived from an absorption peak in the frequency sweep. The method measures the energy missing at the oscillation frequency.
Typically, a function generator inserts a sine wave signal into a coaxial cable. At the location of the measurement, a T-splitter device splits the signal into a part going to the primary coil/transceiver coil and the part going to a receiver for carrying out the spectral analysis. The combination of primary coil and measurement coil in an area being measured absorbs a certain amount of energy at a certain frequency and this absorbed fraction is temperature-dependent. Whatever is left is supposed to reach the receiver. This method uses flux coupling between the coils as in a transformer. However, the method suffers from a low signal to noise ratio and from microphony caused by improper impedance matching.
Actually, only a part of the returning signal from the transceiver coil reaches the analyzing circuit of the receiver whereas another part returns into the generator and leads to a low signal to noise ratio.
The primary coil of the sensor is essentially a loop and thus a balanced-type electrical system, whereas the coaxial standard cable is unbalanced. The inherent issue is mismatching of the impedance of the system which causes microphony, i.e. the dependence on the outer circumstances such as vibrations or tolerances such that the output values will largely change. The microphony is mainly caused by reflected current flowing in the outer skin of a cable that can be influenced by touch or motion. This results in difficulties for calibration and reproducibility.